Background: In the earlier randomized controlled Physical Activity during Cancer Treatment (PACT) study, we found beneficial effects of an 18-week supervised exercise program on fatigue in patients with newly diagnosed breast or colon cancer undergoing adjuvant treatment. The present study assessed long-term effects of the exercise program on levels of ... read more fatigue and physical activity 4years after participation in the PACT study. Methods: The original study was a two-armed, multicenter randomized controlled trial comparing an 18-week supervised exercise program to usual care among 204 breast cancer patients and 33 colon cancer patients undergoing adjuvant treatment. Of the 237 PACT participants, 197 participants were eligible and approached to participate in the 4-year post-baseline measurements, and 128 patients responded. We assessed fatigue and physical activity levels at 4years post-baseline and compared this to levels at baseline, post-intervention (18weeks post-baseline), and at 36weeks post-baseline. Results: Intention-to-treat mixed linear effects model analyses showed that cancer patients in the intervention group reported significantly higher moderate-to-vigorous total physical activity levels (141.46min/week (95% confidence interval (CI) 1.31, 281.61, effect size (ES)=0.22) after 4years compared to the usual care group. Furthermore, cancer patients in the intervention group tended to experience less physical fatigue at 4years post-baseline compared to the usual care group (-1.13, 95% CI-2.45, 0.20, ES=0.22), although the result was not statistically significant. Conclusion: Patients with breast or colon cancer who participated in the 18-week exercise intervention showed significant higher levels of moderate-to-vigorous total physical activity levels and a tendency towards lower physical fatigue levels 4years post-baseline. Our result indicate that exercising during chemotherapy is a promising strategy for minimizing treatment-related side effects, both short and long term. show less